This invention involves a dark, neutral gray colored glass that has low luminous transmittance and low total solar energy transmittance. Although not limited to a particular use, the glass of this invention exhibits a combination of properties that make it highly desirable for use in sun roofs, particularly vehicle sun roofs. These properties include low visible light transmittance to reduce glare, low total solar energy transmittance to reduce heat gain in the interior of the enclosure, a neutral gray color for the sake of coordinating with a wide range of interior and exterior finish colors, and a composition compatible with flat glass manufacturing methods.
In the past, gray colored heat absorbing glasses often relied on the inclusion of nickel as a chief coloring agent. But avoiding the inclusion of nickel is desirable because the presence of nickel during the melting process sometimes leads to the formation of nickel sulfide stones in the glass. Although the nickel sulfide stones are nearly invisible and cause no harm to the glass under normal conditions, the high coefficient of thermal expansion of nickel sulfide can cause thermally induced stresses sufficient to fracture a glass sheet having a nickel sulfide stone. This is a particular problem in applications like automobile sun roofs where tempering of the product is desired but the presence of nickel sulfide stones can produce an unacceptably high rate of thermal breakage during tempering or thereafter. Some prior art gray glass having nickel as a major colorant also has the disadvantage of undergoing a color shift when thermally tempered.
A typical prior art dark gray glass composition is the following, in which nickel is relied on for the gray color:
______________________________________ SiO.sub.2 72.90% by weight Na.sub.2 O 13.70 K.sub.2 O 0.03 CaO 8.95 MgO 3.90 Al.sub.2 O.sub.3 0.10 SO.sub.3 0.27 Fe.sub.2 O.sub.3 0.060 CoO 0.015 NiO 0.095 ______________________________________
Another nickel-containing gray glass composition is disclosed in U.S. Reissue Pat. No. 25,312 (Duncan et al.). The luminous (visible) transmittances for the examples in that patent are higher than desired for the glass of the present invention.
Attempts have been made to produce nickel-free gray glass as shown in U.S. Pat. No. 3,723,142 (Kato et al.) and British Patent Specification 1,331,492 (Bamford). In both of these patents the glass is more transparent than is considered suitable for the sun roof type of application to which the present invention is directed.
Another attempt at nickel-free gray glass is disclosed in U.S. Pat. No. 4,104,076 (Pons) where, instead of nickel, Cr.sub.2 O.sub.3 or UO.sub.2 are required, both of which have their own drawbacks. Chromium compounds are difficult to melt and present disposal problems, and therefore are preferably avoided. Uranium oxide is costly and has a weak coloration effect, requiring relatively large quantities. Although broad ranges for the coloring agents are disclosed in that patent, all of the examples have colorant concentrations that would not produce the particular combination of properties desired here.
U.S. Pat. No. 3,300,323 (Plumat et al.) also involves an attempt to produce gray glass without nickel. Instead of nickel, this patent's approach requires the inclusion of TiO.sub.2 and optionally MnO.sub.2, both of which present significant drawbacks. A glass composition having significant amounts of TiO.sub.2 is not compatible with the float forming process, by which most flat glass is now produced. This is because the TiO.sub.2 causes a yellow color to form when the glass comes into contact with molten tin in the float process. Glass containing MnO.sub.2 has a tendency to form brown coloration when exposed to ultraviolet radiation, thus making product uniformity difficult to maintain. Additionally, the plural valance states of manganese makes control of the oxidizing conditions in the glass melting operation very critical, which renders control of the color difficult in a manufacturing operation.
Glasses having a color described as "rose-smoke" are produced by the teachings of U.S. Pat. Nos. 2,524,719 (Tillyer) and 2,892,726 (Smith et al.) using iron, cobalt, and selenium as the coloring agents. Not only are these glasses not a neutral gray, but also the large amounts of iron required to lower the transmittance render the compositions extremely difficult to melt on a large scale, continuous basis would be required for the commercial manufacture of flat glass.
The combination of iron, cobalt, and selenium is disclosed as the coloring agent for making bronze colored glass in U.S. Pat. No. 3,296,004 (Duncan).
It would be desirable to be able to make a dark, neutral gray, nickel-free glass with low transmittance, without troublesome constituents, and that is compatible with commercial flat glass manufacturing techniques.